Allow enabling bpdu-forwarding on OVS bridges (SOC-9172)

[zzaimeche]

This change allows users to enable BPDU forwarding on OVS bridges that are used for Neutron provider networks (br-fixed, br-floating). Enabling it will allow the physical network to detect and stop loops if these bridges are accidentally misconfigured. It is turned off by default.

More context in:

https://bugzilla.suse.com/show_bug.cgi?id=1133804

(Backported from master, 26878c1a32fedaa73cf1cc880ca47085cb90aac8 )

[zzaimeche]

Not sure of the backport process, hope this is right!

[JanZerebecki]

Backport of https://github.com/crowbar/crowbar-core/pull/1900